Rotary machining devcices such as lathes and grinding machines are customarily equipped with a circle of radially adjustable chuck jaws (of hardened steel) located to grasp and center the workpiece. The jaws are mounted on guideways and opened and closed by simultaneous movement along a helical scroll or track. As such jaws are frequently moved in locating and removing a sequence of workpieces, they gradually become loose in the track and such sloppy fit may allow (or compell) the jaws to tilt unequal amounts when subsequently tightened against the workpiece; the result is that the workpiece and the lathe center or the machining which is performed on the work are not in true axial alignment, although such variance is so small as not to be visually discernable.
For working on material which is easily scratched or marred, such as aluminum workpieces, soft jaws are customarily fastened to the permanent jaws and, to the extent necessary, their contact faces or ends are turned or bored to conform to a (usually cylindrical) workpiece. In such situation, the minute losseness or misalignment of the permanent chuck jaws is magnified in the functional position assumed by the soft jaws.
As a precaution, sometimes a "spider" may be centrally inserted at the base of the chuck; this is a flat disk or plate having a radially extending arm pressed outward against each jaw so as (hopefully) to keep them equally tensioned. In place of the spider, a cammed spacer plate may be provided, as described in U.S. Pat. No. 3,104,474. However, these devices lodged at the base of the jaws do not necessarily correct or equalize the ultimate tilt or eccentricity because (a) they exert much less pressure than that of the jaws when finally clamped tight against a workpiece, and (b) they do not engage the jaws at the outer or free ends which is the area of greatest tilt or misalignment due to leverage extending outward from the base of the jaw. Although the amount of misalignment may be on the order of a few ten thousandths of an inch, such lack of precision may cause the work to be rejected. Eventually the situation is handled by discarding the inaccurate workpieces (when finally checked) and/or by installing a new chuck assembly. Clearly this is not a desirable way of handling the problem.